In an aerial tanker airplane used by the military known as the Boeing KC-135, a flying refueling boom is supported from the airplane about a fixed vertical axis for free pivotal movement in a sideward direction or in azimuth; and the boom is also supported for free pivotal movement about a lateral axis for up-and-down movement or in elevation. The means for moving the boom about these axes, is a pair of aerodynamic surfaces formed in a Vee and known as ruddevators. When the ruddevators are moved collectively to decrease their angle-of-attack, the boom will be moved downwardly; and a differential change in the angle-of-attack of the ruddevators will move the boom sidewardly.
Included in the ruddevator control system is a pantographing cable system, similar to that of a drafting machine, which functions to automatically position the ruddevators when the boom is moved by other than the boom operator's control stick; e.g., when engaged with a receiver airplane and the receiver airplane moves the boom in elevation, the ruddevators will automatically pantograph collectively until they are approximately parallel to the airstream; thereby, alleviating the air loads on the boom without any control input from the boom operator. Also, when the receiver airplane pulls the boom off to one side, the ruddevators will pantograph differentially to alleviate the air loads.
The present aerial tankers like the Boeing KC-135, as well as the previous tankers like the Boeing KB-29 and KC-97, have used a flying boom type of aerial refueling with considerable success. However, limitations in the aerodynamic performance of the boom become apparent when it is used at the ever increasing speeds and altitudes required to refuel the modern high speed military aircraft. In order to provide a flying boom system of aerial refueling that would permit a hook-up between the tanker airplane and the receiver airplane, at the normal cruise speed and altitude of the receiver airplane, there must be adequate aerodynamic control forces available from the airfoil surfaces activating the boom, in order to position it throughout a space envelope large enough for the receiver airplane to remain within, and with a reasonable effort on the part of the pilot of the receiver airplane and the boom operator. For the present known tanker airplanes like the Boeing KC-135, the increase in airspeed or Mach number of the tanker airplane to that required for optimum refueling of the present day military interceptor type airplane, could reduce the boom maneuvering envelope to the point where the refueling operation would be nearly impossible; e.g., if due to restrictions on the operation of the boom, the refueling operation requires that it be done at lower airspeeds, a considerable loss in altitude as well as true airspeed would be necessary for the receiver airplane. This could result in the increased vulnerability of both aircraft to enemy interception as well as the more severe weather conditions at the lower altitudes which could complicate the rendezvous and refueling hook-up operation.
One of the inherent aspects of the present known KC-135 ruddevator control system, is that when the boom is moved all the way over to one side of the azimuth envelope, there is quite a bit of air drag generated by the boom; and this results in a decrease in the aerodynamic control force effectiveness of the ruddevators in maintaining that extreme azimuth position. Also, at this extremity of the azimuth envelope, the wake flow from the boom blanks out the air flow over one of the ruddevators. It could be said, that with the KC-135 fixed vertical and horizontal hinge geometry, the boom yaws right out of control power, i.e., it can not be flown nor driven by the ruddevators to the extremities of the desired envelope.
A more detailed explanation of the manner in which this control system operates is disclosed in U.S. Pat. No. 2,960,295 to Schulze.